Charlie N. Barron

The Modular Ocean Data Assimilation System (MODAS)

The Modular Ocean Data Assimilation System (MODAS) is used by the U.S. Navy for depiction of three- dimensional fields of temperature and salinity over the global ocean. MODAS includes both a static climatology and a dynamic climatology. While the static climatology represents the historical averages, the dynamic cli- matology assimilates near-real-time observations of sea surface height and sea surface temperature and provides improved temperature and salinity fields. The methodology for the construction of the MODAS climatology is described here. MODAS is compared with Levitus and Generalized Digital Environmental Model climatologies and with temperature and salinity profiles measured by SeaSoar in the Japan/East Sea to illustrate MODAS capabilities. MODAS with assimilated remotely sensed data is able to portray time-varying dynamical features that cannot be represented by static climatologies.

The Cape Cauldron: A regime of turbulent inter-ocean exchange

Combining in-situ Lagrangian intermediate depth velocity measurements from the KAPEX (Cape of Good Hope Experiments) float program with sea-surface height data, this study reviews the inter-ocean exchange mechanisms around southern Africa. In the southeastern Cape Basin, a highly energetic field of coexisting anticyclonic and cyclonic eddies is documented. Agulhas Rings of typically 200 km diameter are observed to merge, split, deform, and to reconnect to the Agulhas Retroflection. Concomitant, slightly smaller cyclones are observed to drift across the northwestward migration path of the Agulhas Rings. These cyclones, with typical diameters of 120 km, are formed within the Cape Basin along the African shelf, inshore of the Agulhas Current, and in the subantarctic region south of Africa. The data suggest the annual formation of 3–6 long-lived Agulhas Rings that eventually cross 5°E longitude, while approximately twice the number of rings occur in the southeastern Cape Basin. Within this region, cyclones outnumber anticyclones by a factor of 3:2. Both cyclones and anticyclones extend through the upper thermocline into the intermediate depth layer. Mean drifts of anticyclones are 3.8±1.2 cm s−1 to the northwest, while cyclones follow a west–southwestward route at 3.6±0.8 cm s−1. Transport estimates suggest that the intermediate depth layer in the southeastern Cape Basin is primarily supplied from the east (approximately 9 Sv), with minor direct inflow from the Atlantic to the west and south. Cyclone/anticyclone interaction is surmised to result in vigorous stirring and mixing processes in the southeastern Cape Basin, which necessitates a review of the traditional concept of Indo-Atlantic inter-ocean exchange. We propose to limit the concept of “isolated Agulhas Rings embedded in a sluggish Benguela Drift” to the northwestern Cape Basin and beyond, while linking this regime to the Agulhas Retroflection proper through a zone of turbulent stirring and mixing in the southeastern Cape Basin, named for the first time the “Cape Cauldron” hereinafter.

Path and variability of the Agulhas Return Current

The combined analysis of hydrographic, kinematic, and dynamic data collected during the 1997–1999 KAPEX (CAPe of Good Hope EXperiments) reveals a quasi-stationary meandering pattern of the Agulhas Retroflection Current east and upstream of the Southwest-Indian Ridge. The current meanders between 38°S and 40°S in a spatially and temporally continuous fashion and has a core width of approximately Full-size image (<1 K) with an associated transport of Full-size image (<1 K) in the upper Full-size image (<1 K). Peak surface velocities decrease from Full-size image (<1 K) near the Agulhas Retroflection to Full-size image (<1 K) around 32°E. Meander troughs (northward extremes) are found predominantly near 26.8°E, 32.6°E and 38.9°E, while crests (southward extremes) are located with high probability near 29.7°E, 35.5°E and 42.9°E, resulting in a typical wavelength of Full-size image (<1 K). Cold eddies are shed along the northern boundary of the current from meander troughs into the recirculation regime between the Agulhas Current proper and the Agulhas Return Current. Strongest cyclonic eddies are preferably shed in austral autumn. The cyclonic eddies so formed propagate westward at an average phase-speed of Full-size image (<1 K), with, however, a variability of at least the same magnitude. Subsequently, the cyclones are absorbed by the next meander trough located upstream and to the west of the shedding trough.

Sea Surface Height Predictions from the Global Navy Coastal Ocean Model during 1998–2001*

A ?8 global version of the Navy Coastal Ocean Model (NCOM), operational at the Naval Oceanographic Office (NAVOCEANO), is used for prediction of sea surface height (SSH) on daily and monthly time scales during 1998‐2001. Model simulations that use 3-hourly wind and thermal forcing obtained from the Navy Operational Global Atmospheric Prediction System (NOGAPS) are performed with/without data assimilation to examine indirect/direct effects of atmospheric forcing in predicting SSH. Model‐data evaluations are performed using the extensive database of daily averaged SSH values from tide gauges in the Atlantic, Pacific, and Indian Oceans obtained from the Joint Archive for Sea Level (JASL) center during 1998‐2001. Model‐data comparisons are based on observations from 282 tide gauge locations. An inverse barometer correction was applied to SSH time series from tide gauges for model‐data comparisons, and a sensitivity study is undertaken to assess the impact of the inverse barometer correction on the SSH validation. A set of statistical metrics that includes conditional bias (Bcond), root-mean-square (rms) difference, correlation coefficient (R), and nondimensional skill score (SS) is used to evaluate the model performance. It is shown that global NCOM has skill in representing SSH even in a free-running simulation, with general improvement when SSH from satellite altimetry and sea surface temperature (SST) from satellite IR are assimilated via synthetic temperature and salinity profiles derived from climatological correlations. When the model was run from 1998 to 2001 with NOGAPS forcing, daily model SSH comparisons from 612 yearlong daily tide gauge time series gave a median rms difference of 5.98 cm (5.77 cm), an R value of 0.72 (0.76), and an SS value of 0.45 (0.51) for the ?8 free-running (assimilative) NCOM. Similarly, error statistics based on the 30-day running averages of SSH time series for 591 yearlong daily tide gauge time series over the time frame 1998‐2001 give a median rms difference of 3.63 cm (3.36 cm), an R value of 0.83 (0.85), and an SS value of 0.60 (0.64) for the ?8 free-running (assimilated) NCOM. Model‐ data comparisons show that skill in 30-day running average SSH time series is as much as 30% higher than skill for daily SSH. Finally, SSH predictions from the free-running and assimilative ?8 NCOM simulations are validated against sea level data from the tide gauges in two different ways: 1) using original detided sea level time series from tide gauges and 2) using the detided data with an inverse barometer correction derived using daily mean sea level pressure extracted from NOGAPS at each location. Based on comparisons with 612 yearlong daily tide gauge time series during 1998‐2001, the inverse barometer correction lowered the median rms difference by about 1 cm (15%‐20%). Results presented in this paper reveal that NCOM is able to predict SSH with reasonable accuracies, as evidenced by model simulations performed during 1998‐2001. In an extension of the validation over broader ocean regions, the authors find good agreement in amplitude and distribution of SSH variability between NCOM and other operational model products.

Early evolution of an Agulhas Ring

Abstract Rings shed at the Agulhas retroflection are an integral part of interoceanic exchange south of Africa. There is clear evidence of westward ring translation from the northern Cape Basin across the South Atlantic Ocean. Early ring development and translation from the southern to the northern Cape Basin, however, are obscured by an intensely variable kinematic field close to the spawning site. In this study unique in situ observations, obtained in March to September 1997, are analyzed to improve the understanding of the early development of a juvenile Agulhas Ring. In March the ring was surveyed near 37°S, 16°E, approximately 4 months after its generation. Its strength and size were in the upper range typical for Agulhas Rings, and its trapping depth extended down to at least 1600 dbar according to geostrophic velocities and RAFOS trajectories in the ring. Between March and September the ring propagated in a general northwestward direction; however, RAFOS trajectories and MODAS sea-surface steric height fields revealed a large variability of the translation speed ( 3 cm s −1 to more than 20 cm s −1 ) and direction. In September 1997, the mature ring was examined near 31°S, 9°E. By this time, its available heat and salt anomaly were reduced by about 30% and its available potential energy was reduced by about 70%. This indicates that a significant loss of the ring characteristics occurred on the way from the southern to the northern Cape Basin. One-third of this loss is due to changes at intermediate depth (between 800 and 1600 m ).

Comparisons of monthly mean 10 m wind speeds from satellites and NWP products over the global ocean

Received 31 December 2008; revised 15 June 2009; accepted 19 June 2009; published 27 August 2009. [1] The accuracy of wind speed at 10 m above the sea surface from two satellite and three numerical weather prediction (NWP) products is investigated over the global ocean. Rain-free equivalent neutral winds from the Quick Scatterometer (QuikSCAT) are converted to stability-dependent winds to be consistent with those from NWP products and are taken as truth in comparisons to winds from other products. Quantitative statistical analyses presented at each grid point over the global ocean reveal that monthly winds from NWP products have almost perfect skill relative to those from QuikSCAT winds during the 3-year common period (September 1999 to August 2002). Exceptions occur in tropical regions and high southern latitudes. Wind speeds adjusted to 10 m at many moored buoys located in different regions of the global ocean further confirm the accuracy of monthly NWP winds, giving RMS difference of 1.0 m s � 1 based on 1281 monthlong time series. The satellite-based QuikSCAT winds agree with buoy winds relatively better than NWP products. While there is good agreement among wind products on monthly timescales, large differences (>3 m s � 1 and more) in NWP winds are found in comparison to QuikSCAT winds on shorter time intervals at high latitudes. Daily means of sensible and latent heat fluxes based on NWP winds can therefore differ as much as 100 W m � 2 in comparison to those based on QuikSCAT winds. In general, NWP wind-based sensible and latent heat fluxes are more similar to their QuikSCATwind-based counterparts in tropical regions and midlatitudes.

Assessment of Data Assimilative Ocean Models in the Gulf of Mexico Using Ocean Color

Abstract : This paper illustrates the value of SeaWiFS ocean color imagery in assessing the ability of three data-assimilative ocean models (configured in five prediction systems) to map mesoscale variability in the Gulf of Mexico (i.e., the Loop Current and associated warm and cold eddies) and in helping to diagnose specific strengths and weaknesses of the systems. In addition, the study clearly illustrates that biological responses of the surface waters are strongly linked to the physical events and processes.

Accuracy of 10 m winds from satellites and NWP products near land‐sea boundaries

Microwave/Imager. Large biases (e.g., >3 m s 1 ) may exist in NWP products near the land-sea boundaries, because wind speeds from the uniformly gridded global fields are generally at a spatial scale too coarse to appropriately define the contrast between water and land grid points. This so-called land contamination of ocean-only winds varies, and typically depends on the extent of the land-sea mask. A creeping sea-fill methodology is introduced to reduce errors in winds. It is based on the elimination of land-corrupted NWP grid points and replacement by adjacent, purely over-ocean values. In comparison to winds from many moored buoys, the methodology diminishes RMS errors (from >4 m s 1 to <1 m s 1 ) for NOGAPS and ERA-40. The creeping sea-fill is not advised for NCEP winds which have low contrast between land and sea points, thereby resulting in little impact from the land contamination.

A comparison of in-situ float velocities with altimeter derived geostrophic velocities

Abstract Satellite borne altimetric data are of increasing prominence for assimilation in ocean circulation models and interpretation of localized in-situ measurements. Physically, geo-referenced sea-surface height (SSH) data products are mostly referenced relative to a long-term SSH mean, and consequently called SSH-anomalies. The Modular Ocean Data Assimilation System (MODAS) adds climatological SSH fields to provide space–time interpolated absolute steric SSH fields. This, in theory, should provide realistic geostrophic surface velocities and flow patterns, including quasi-permanent features such as western boundary currents or free jets. This study compares such data for the wider Agulhas Retroflection region with co-located, simultaneous velocity measurements from Cape of Good Hope Experiments (KAPEX). KAPEX used ship-borne Acoustic Doppler Current Profiles (ADCP) and neutrally buoyant RAFOS (Ranging and Fixing of Sound) floats at intermediate depths to obtain in-situ velocity data. Correlation coefficients of MODAS-2D geostrophic and RAFOS subsurface flow directions fall between 0.8 and 0.9 with a typical error less than 0.05. The high correlation suggests that MODAS-2D provides a correct depiction of anticyclonic/cyclonic flow patterns in this region, making it a useful tool to describe the Agulhas Retroflection. Root-mean-square differences between velocities as measured by the various data sources rage between 20 and 30 cm s −1 , lying between the natural variability observed for the intermediate and surface layers. Decreasing slope parameters of linear regressions between MODAS, RAFOS and ADCP velocities reflect the baroclinic velocity shear. Slope equals 1 at surface and decreases to 0.4 at depths below 1000 m . Offsets of linear regression of these fits are not significantly different from zero, except for the zonal component in the Agulhas Return Current (5– 10 cm s −1 ). This discrepancy suggests a missing meridional gradient in this regions climatological signal that is added to the SSH anomaly field within MODAS.

Temperature versus salinity gradients below the ocean mixed layer

Abstract : We characterize the global ocean seasonal variability of the temperature versus salinity gradients in the transition layer just below the mixed layer using observations of conductivity temperature and depth and profiling float data from the National Ocean Data Center s World Ocean Data set. The balance of these gradients determines the temperature versus salinity control at the mixed layer depth (MLD). We define the MLD as the shallowest of the isothermal, isohaline, and isopycnal layer depths (ITLD, IHLD, and IPLD), each with a shared dependence on a 0.2 deg C temperature offset. Data are gridded monthly using a variational technique that minimizes the squared analysis slope and data misfit. Surface layers of vertically uniform temperature, salinity, and density have substantially different characteristics. By examining differences between IPLD, ITLD, and IHLD, we determine the annual evolution of temperature or salinity or both temperature and salinity vertical gradients responsible for the observed MLD. We find ITLD determines MLD for 63% and IHLD for 14% of the global ocean. The remaining 23% of the ocean has both ITLD and IHLD nearly identical. It is found that temperature tends to control MLD where surface heat fluxes are large and precipitation is small. Conversely, salinity controls MLD where precipitation is large and surface heat fluxes are small. In the tropical ocean, salinity controls MLD where surface heat fluxes can be moderate but precipitation is very large and dominant.